1. Field
The present invention relates to light source and light beam scanning units in use for image recording systems such as a laser printer, digital duplication machine, facsimile apparatus and other similar systems.
2. Discussion of the Background
In many recent developments of digital duplication machines, laser printers and facsimile apparatuses, more attention has been focused on the increase in speed and image quality of information recording, among others. In order to materialize such improved capabilities, it is desirable for light source and light beam scanning units to have high-precision and stability of aligned light beam axes, ease of adjustment, and multiple-beam scanning capability.
The light source and scanning unit generally includes at least a semiconductor laser device as light source, and coupling lenses for coupling the light beams emanated from the source by properly transmitting to the following optical units as a parallel, converging or diverging beam as prescribed.
Such light source unit is required to have basic requirements of optical properties such as the directivity of laser beams emanated from the source (i.e., optical axis characteristics), and the property of the beams of being parallel, suitably converging or diverging (i.e., coupling characteristics).
The light source unit, therefore, has to retain proper positions with high precision of light emitting points and coupling lenses, with respect to each other at the time of initial adjustment, after variation with time and through the changes in ambient conditions.
An exemplary light source unit included in a scanning unit is disclosed in Japanese Laid-Open Patent Application No. 8-7294, including at least an LED 101 and lens holder 102 shown in FIGS. 15 and 16.
FIG. 15 is a perspective view of the lens holder 102 and a collimator lens 104 and FIG. 16 is a cross-sectional view taken along the line I-I of the structure of FIG. 15.
This disclosure relates primarily to a method for holding the collimator lens 104 through press-holding against an L-shaped abutting face 105 or a V-shaped groove by a pressing means 106 without known methods such as, for example, applying adhesive agents. This method, however, appears to fall short of offering satisfactory solutions to several points which follow.
{circle around (1)} Difficulties caused by the difference in coefficients of thermal expansion: To be more specific, the package portion of LED 101 is generally made of iron alloy materials, while the kind of materials for forming the collimator lens 104 is glass or resinous materials in general.
In addition, a light source supporting portion 103 and lens holder 102 as the supporting members for the LED source 101 and collimator lens 104, respectively, are often made of aluminum or resinous materials.
Since the coefficients of linear thermal expansion are different each other for the materials used in the package portion of LED 101, collimator lens 104, source supporting portion 103 and lens holder 102, a deviation may arise in the relative position of emitting source point SEC and the optical axis L of the collimator lens 104.
This deviation is exemplified by several changes in the position against the point A shown in FIG. 16.
That is, since the amount of positional change (i.e., elongation or shrinkage) in the secondary scanning direction (z-axis) in the vicinity of the light source (LED) 101, which is denoted by Δt1, differs from the positional change in the vicinity of the collimator lens 104, which is denoted by (Δt2+Δt3), there gives rise to the deviation in the relative position of, or the discrepancy between, the emitting source point SEC and the optical axis L of the collimator lens 104, thereby resulting in the relationΔt1≠Δt2+Δt3.
{circle around (2)} Difficulties in insufficient alignment: As shown in FIG. 17, which depicts as the device of FIG. 15 viewed in the B direction, if the friction of the collimator lens 104 is unduly high against the L-shaped abutting face 105 or V-shaped groove as guiding means 106, or the hardness of the pressing means 106 is not high enough for proper abutting, and the collimator lens 104 cannot completely follow after the guiding means 106. This may result in slant-abutting as shown in FIG. 17, in which the collimator lens 104 is held by being in contact with only two locations but not in a linear manner (denoted by the mark ‘•’), thereby giving rise to rather unstable holding conditions.
As a result, the alignment of the collimator lens 104 may be disturbed by vibration and/or the change in ambient temperatures.
Furthermore, since the direction of the light beams is altered generally not only by the positional deviation but also by the change in the alignment of the collimator lens 104, the direction of light beams emanated presently also change with the variation of the alignment.
{circle around (3)} Furthermore, when the pressing member 106 for pressing collimator lens 104 is formed of the shape shown in FIG. 18, which depicts the device of FIG. 15 viewed in the C direction, and when the coefficient of linear thermal expansion of the pressing member 106 is larger than that of the collimator lens 104 and lens holder 102, the former becomes more elongated with the increase in temperature. This generates a force denoted by the arrow D in FIG. 18, which acts onto the collimator lens 104, and a concomitant component force along the abutting face 105. As a result, the position of the collimator lens 104 may cause a change in the direction of light beams.
As to the means for the aforementioned increase in the output capability of image forming apparatus, several methods will be cited such as (1) increasing the speed of translation (or rotation) of image bearing member, and (2) utilizing a plurality of image bearing members.
For implementing the improvement according to method (1), the speed of light beam scanning unit for writing image data into the image forming member has to be increased, and that method for increasing the speed of writing image data into the image forming member, in turn, conceivably requires the increase in (i) rotation speed of the polygon scanner as a beam deflecting means, and (ii) the number of light beams for use in writing.
However, several difficulties are encountered such as insufficient durability of the driving motor, undue noise, vibration, and/or speed of laser beam modulation with respect to the method (i), and the need for a light source unit emanating multiple laser beams with the method (ii).
As an exemplary multiple beam source unit, the use of a semiconductor laser array is mentioned, which is provided with a plural light emitting points (or light emitting channels) in one package.
Such semiconductor laser array, however, is rather expensive for practical application at present since it is difficult to increase the channels for the reasons of fabrication process, remove undue effects from thermal and electrical cross-talk, and materialize shorter emission wavelengths.
In addition, present market price per unit channel of the semiconductor laser array rather increases with the increase of the number of light emitting points in a package. For example, the price of one semiconductor laser array with four light emitting points is considerably higher than four pieces of single-beam laser diodes when used for the scanning light source.
Accordingly, further light source units or multiple-beam scanning units have been disclosed, for example, using plural single-beam laser diodes (or diodes each having a relatively small number of light emitting points) with emitted light beams to be subsequently synthesized and scanned, as described in Japanese Laid-Open Patent Application No. 10-284803.
In the above noted multiple-beam scanning units, the relative position of, or the distance between, the plural beams along the vertical scanning direction on image forming member is important. Namely, if the deviation in the relative position from the prescribed value arises, this deviation will show up repeatedly in picture images, thereby causing faulty images accompanied by periodic undue stripes.
Therefore, it is of primary importance to reduce the deviation in the relative position of the light emitting points and coupling lens within the prescribed value.
The deviation in relative position of the plural beams along the horizontal scanning direction on image forming member can be corrected with relative ease by adjusting the activation timing of light emission following the synchronization detection by a synchronization detector means.
In contrast, the deviation along the vertical scanning direction is more difficult in general, whereby a more complicated system for correcting thereof is preferred.
In addition, in a beam scanning unit disclosed in Japanese Laid-Open Patent Application No. 7-181410 (which is cited in the Application No. 10-284803), there is provided a fitting allowance of approximately from 0.01 to 0.03 mm between the lens holder and collimator lens, and the lens holder and fitting hole on a flange, respectively, to be filled with, and subsequently fixed by hardening an adhesive agent.
The adhesive agent has, in general, a relatively large change in shape and volume through shrinking during hardening, variation with time, and the change of ambient conditions. Moreover, the noted allowance may differ from one unit to another due to dispersion during fabrication, which results in further variation of the amount of adhesive agent. These factors may add up to the change in relative position between plural collimator lens through shrinking during hardening, variation with time, and the change of ambient conditions. As a result, this causes the change or deviation in the position of plural light beams relative to each other, which is illustrated with two light beams shown in FIG. 19.
Also in the beam scanning unit disclosed in the Japanese Application No. 7-181410, the aligning steps are carried out for fixed LEDs through the adjustment of relative position of collimator lenses by tri-axially displacing each lens and the subsequent fixing with adhesive agent.
When a certain allowance for the adjustment is taken into consideration, a clearance of approximately 0.2 mm is needed between collimator lens and lens holder, to be subsequently filled with the adhesive. Because of this rather thick adhesive layer and following positional adjustment, a deviation in the position of collimator lens may result as much as about ±0.1 mm.
As a result, the overall thickness of adhesive layer may amount to be in the range of from 0.1 to 0.3 mm, thereby causing a large dispersion in finished units. Therefore, relatively large dispersion (or fluctuation) in the position of collimator lens also results from one unit to another.
In addition, a fitting face of the lens holder against the collimator lens is formed as a circle concentric with the collimator lens. If the collimator lens 104 is fixed to a position deviated in the vertical scanning direction (z-axis) as shown in FIG. 19, a deviation arises in the thickness of adhesive agent 107 in that direction, which results in with relative ease the positional deviation in collimator lens 104 in the vertical scanning direction due to the expansion and contraction of the layer of adhesive agent caused by ambient conditions.
As a result, because of the positional deviation of collimator lens 104 in the vertical scanning direction and the noted relatively large dispersion in the position of collimator lens from one unit to another, the deviation in relative position of two beam spots formed on the image forming member may increase.
Incidentally, the positional relation between the collimator lens 104 and lens holder fixed by the adhesive agent 107 is shown in FIG. 19.
Furthermore, when the light source unit of FIG. 15 is utilized as one of the beam sources to be synthesized using a prism, the discrepancy may arise, due to the aforementioned methods {circle around (1)}, {circle around (2)} and {circle around (3)}, in relative position between emitting source point SEC and optical axis L of the collimator lens 104 along the y- or z-direction, and the direction of the light beam emanated and transmitted through the collimator lens 104 changes, whereby the relative position of plural beams following the beam synthesis is unduly affected.
As a result, the deviation in relative position of two beam spots formed on the image forming member results and this may degrade picture image qualities of image outputs.
In addition, as the means for increasing output speed for image forming apparatus, there can be cited the case of color image forming apparatus incorporating four image forming members along the line of the aforementioned method {circle around (2)}, which requires plural light beams for writing images of the corresponding number.
In such case, when the position of light beams to be scanned, for writing images, over respective image forming members deviates relative from each other due to the change over time and/or ambient conditions, the precise overlap of printed colors may not be able to achieve, to thereby result so-called shear in color printing.